The proposed research addresses itself to several interrelated aspects of regulation of myogenesis during the growth and differentiation of embryonic cardiac and skeletal muscle cells. The first one is concerned with the question whether or not muscle-specific gene expression is regulated in a subtle manner by mRNA-associated proteins. Recent work in our laboratory has shown that the mRNAs coding for two muscle-specific proteins, actin and myosin heavy chain, are present in embryonic skeletal muscle cells as cytoplasmic messenger ribonucleoprotein (mRNP) particles. Furthermore, the size and availability of the cytoplasmic pool of mRNA-associated proteins may act as a subtle regulator of translation in embryonic muscle cells and by analogy other differentiated cells. However, the biochemical nature of the regulatory processes involving mRNP particles remains to be defined. We now plan to study whether or not the mRNP particles are involved in the regulation of one or more of the many possible aspects of mRNA metabolism in embryonic muscle cells such as transport, stabilization and initiation of translation, which in turn will affect the overall translation process. The second aspect of the proposed studies deals with the question whether or not the polymorphic forms of myofibrillar proteins which appear at different stages of muscle development are due to exogenous factors such as activity pattern of the fiber, neural influence, etc. It is hoped that the results of these studies will be helpful in understanding the molecular basis of the regulation of myogenesis and in probing for possible alterations of such regulatory mechanisms in pathological states of the muscle tissue such as muscular dystrophy and diseases of neuromuscular origin and cardiac hypertrophy.